Fast neutron nuclear reactors are generally cooled with a liquid metal such as sodium and comprise a vessel closed by a horizontal, very thick slab and filled with the coolant liquid sodium in which the core of the reactor is immersed. Disposed above the core is an assembly termed a "core cover plug" which supports the sheath tubes of the control rods and the whole of the instrumentation of the core comprising in particular the tubes taking off sodium at the outlet of the assemblies and the thermocouples for measuring the temperature in the assemblies of the core. This core cover plug also ensures, by its lower part, the deflection of the stream of sodium issuing from the core to direct it to the intermediate exchangers at the periphery of the vessel.
The core cover plug is constituted by a generally cylindrical barrel having a vertical axis whose upper part is fixed to a support plate bearing on a rotating plug mounted on the slab of the reactor. The rotating plug has an opening for the passage of the cover plug of the core; the rotating plug is mounted to be rotatable about a vertical axis on the slab closing the vessel of the reactor.
The cover plug of the core also includes an group of vertical tubes of which some are adapted to receive the control rods of the reactor and others the instrumentation devices for effecting neutronic measurements and temperature measurements in the core and for taking off the sodium at the outlet of the fuel assemblies constituting the core. The lower end of the barrel of the core cover plug is located just above the upper part of the assemblies and receives a highly apertured transverse plate which is fixed to the cylindrical barrel. The openings extending through this apertured plate permit both the passage, of the sodium for cooling the assemblies, and assured maintenance and guiding of the tubes receiving the control rods and of the instrumentation tubes.
Above the apertured plate, another plate which is transverse with respect to the cylindrical barrel permits the deflection and the dispersion of the sodium issuing from the core. This plate has a first group of bores in which the instrumentation tubes are fixed, a second group of bores through which extend the guide tubes of the control rods, and a few apertures for the passage of sodium to ensure an equilibrium of level and pressure between the part inside and the part outside the barrel of the core cover plug. The deflection plate therefore has, in contrast with the apertured plate, a very small total section of the passage for the sodium. The sodium is therefore laterally deviated by this plate and reaches the heat exchangers immersed in the vessel of the reactor, after having passed through the barrel of the core cover plug through a group of openings provided in this barrel.
In known and presently-used core cover plugs, the transverse deflection plate is fixed to the cylindrical barrel in such manner that these elements are subjected to very high stresses which are prejudicial to a long service life.
Indeed, the lower part of the core cover plug is subjected to extremely high stresses due to the hydrodynamic flow conditions of the sodium and to the thermal conditions prevailing in this region of the vessel of the reactor. The temperature, which corresponds to the temperature of the sodium issuing from the core, is high in this region and may undergo very rapid variations, in particular in the event of urgent shut-downs of the reactor.